1. Field of the Invention
The present invention relates to a solder bump transfer sheet, a method for producing the same, a method for fabricating a semiconductor device, and a method for fabricating a printed board. More particularly, the present invention relates to a method for producing a solder bump sheet used advantageously for connection of a semiconductor chip to a board of a package.
2. Description of Related Art
Solder bumps are used for connection between a semiconductor device and a circuit board and connection between a semiconductor chip and a circuit board in order to provide both electrical connection and mechanical connection.
The xe2x80x9csemiconductor devicexe2x80x9d as used herein refers to a package of a semiconductor chip, including not only a package of a single semiconductor chip, but also a package of a plurality of semiconductor chips, such as a multi chip module (MCM). The xe2x80x9ccircuit boardxe2x80x9d as used herein includes not only a printed board on which a semiconductor device is mounted, but also a wiring board of a package on which a bare chip is mounted by flip chip bonding. The present invention is particularly advantageous when used for formation of narrow-pitch solder bumps. Herein, therefore, solder bumps for flip chip bonding will be described.
Paste printing and ball mounting are conventionally employed as methods for forming solder bumps. However, in paste printing, formation of fine bumps is difficult. It is also difficult to control the height (thickness) of bumps with high precision. In ball mounting, comparatively fine bumps can be formed. However, an expensive mounter is necessary to mount and press solder balls shaped into a perfect sphere (diameter: 0.76 mm xc2x10.02 mm, for example) at predetermined positions. This greatly increases production costs. Moreover, since handling of fine balls with reliability is difficult work, production efficiency decreases.
The present inventor disclosed a method for forming fine solder bumps using a solder bump transfer sheet in Japanese Laid-Open Patent Publication No. 11-312758, in which a film formed by plating is patterned by a photolithographic process, for example. By this method, fine bumps with a reduced variation in height (thickness) can be produced with great efficiency.
However, as a result of further investigation by the present inventor, it has been found that the solder bump transfer sheet disclosed in Japanese Laid-Open Patent Publication No. 11-312758 has a variation in the transferability, or the degree of transfer, of solder bumps. The disclosed transfer sheet has difficulty in transferring a large number of bumps, for example, more than 1000 bumps to a chip or the like with reliability, and this may cause decreases in the yield. This problem is more serious as the integrity of the semiconductor device enhances.
For solder bumps used for connection of a semiconductor chip having a large-capacity DRAM (for example, an ASIC), it is necessary to use a solder material of which the content of an xcex1-ray source is small, to prevent the DRAM from malfunctioning due to an xcex1-ray. Solder material costs for such a low xcex1-ray are high (for example, three times as high as gold). Since the costs are high for this type of semiconductor chip itself, the overall production cost further increases. In consideration of this, improvement of the yield is strongly desired.
An object of the present invention is to provide a solder bump transfer sheet capable of transferring a number of fine solder bumps placed at high density on the transfer sheet with reliability, to provide a method for producing such a solder bump transfer sheet, and provide methods for fabricating a semiconductor device and a printed board using such a solder bump transfer sheet.
The method for producing a solder bump transfer sheet of the present invention includes the steps of: providing a sheet having a chromium oxide layer containing substantially no iron oxide as the outermost surface; and forming a plurality of solder bumps placed in a predetermined pattern on the chromium oxide layer.
Preferably, the sheet is a sheet having a conductive surface, the chromium oxide layer being formed on the conductive surface, and the plurality of solder bumps are formed by electroplating.
Preferably, the sheet is a stainless steel sheet, and the chromium oxide layer is formed by a process including a step of thermally oxidizing the conductive surface of the stainless steel sheet.
Preferably, the chromium oxide layer is formed on an underlying oxide layer containing an iron oxide formed on the surface of the stainless steel sheet, and the thickness of an oxide layer composed of the chromium oxide layer and the underlying oxide layer is 8 nm or more.
Preferably, the step of forming a plurality of solder bumps includes the steps of: forming a resist layer on the chromium oxide layer, the resist layer having a plurality of openings formed in a predetermined pattern; and depositing solder on portions of the chromium oxide layer exposed in the plurality of openings of the resist layer by electroplating.
In a preferred embodiment, the method further includes the step, prior to the step of depositing solder by electroplating, of washing the outermost surface of the stainless steel sheet with an acid aqueous solution.
Preferably, the step of washing the outermost surface is executed so that the thickness of the oxide layer composed of the chromium oxide layer and the underlying oxide layer increases by 5% or more as a result of the acid washing.
Preferably, the step of depositing solder by electroplating is executed at a current density of 0.5 A/dm2 to 4 A/dm2 by a rack plating method.
The solder bump transfer sheet of the present invention has substantially the same structure as that of a solder bump transfer sheet produced by any of the methods for producing a solder bump transfer sheet described above in the prior art.
The method for fabricating a semiconductor device of the present invention is a method for fabricating a semiconductor device including a semiconductor chip mounted on and connected to a wiring board. The method includes the steps of: providing a semiconductor chip having a plurality of pads placed in a predetermined pattern; providing a solder bump transfer sheet having a plurality of solder bumps placed to correspond to the predetermined pattern of the plurality of pads, which is a solder bump transfer sheet having substantially the same structure as that of a solder bump transfer sheet produced by any of the methods for producing a solder bump transfer sheet described above; transferring the plurality of solder bumps of the solder bump transfer sheet to the plurality of pads of the semiconductor chip; and connecting the semiconductor chip to a wiring board via the plurality of solder bumps.
The method for fabricating a printed board of the present invention is a method for fabricating a printed board including interconnections and a plurality of solder bumps formed on a substrate. The method includes the steps of: providing a substrate on which interconnections and a plurality of pads in a predetermined pattern are formed; providing a solder bump transfer sheet having a plurality of solder bumps placed to correspond to the predetermined pattern of the plurality of pads, which is a solder bump transfer sheet having substantially the same structure as that of a solder bump transfer sheet produced by any of the methods for producing a solder bump transfer sheet described above; and transferring the plurality of solder bumps of the solder bump transfer sheet to the plurality of pads on the substrate.